Teaming Heat Pumps with Rooftop Solar Power

Master plumber Dave Ragsdale purges the refrigerant lines of a 12,000-Btu Fujitsu mini-split heat pump. In tandem, the solar panels and heat pumps will relieve the home's existing oil boiler of much of its annual job.

Trends are positive for solar electric power. Panel prices have been dropping for years, while panel output and efficiency has been rising.

Mini-split heat pumps are also riding a wave: Every year, one of the big name-brand mnufacturers leap-frogs the others, posting another jump in efficiency. (The latest winner—for now—in the efficiency arms race is the 9,000-Btu Mitsubishi MSZ-FH09NA, rated at SEER 30 and able to heat at its rated output when outdoor temperatures go as low as 5°F).

For ReVision Energy in Portland, Maine, those two positive trends are the key to a new business model. For several years, the company has been selling Mainers on the idea that the smart way to heat your house is to put solar panels on your roof, and mount a mini-split heat pump on your wall. Some houses only get panels, and some houses only get mini-splits; but as ReVision founder Fortunat Mueller said in February during a three-hour technical presentation at the Building Energy 14 conference in Boston, "To me, the easiest path to net zero—if there are easy paths to net zero—is an on-site, grid-tied solar electric system and some form of electric heat, either resistive or a heat pump. Heat pumps let you cool and heat efficiently with electricity, and electricity is easy to make off the roof." The combination is enough to serve as a stand-alone heating system, if the house is well suited for the strategy. "We are principally a solar business," says Mueller, "but we are excited about the use of heat pumps to leverage solar to get towards net zero, or close to net zero."

For a mini-split system with PV power to take a house to net zero on an annual basis, two things have to happen: The heat pump system has to meet the home's whole heating load, and the photovoltaic array has to put out as much power every year as the heat pump uses to heat the house all winter. That means the house has to consume a relatively modest amount of heat, and it also means there has to be room somewhere—on the roof, or on the ground—for a solar array that will put out enough power to satisfy those needs.

Even in new construction, not every house can make the cut. Mueller says, "We do a lot of projects that are sort of net zero-'ish' buildings, or 'net zero ambition' buildings. They end up something short of net zero, but they're still exceptional buildings with exceptionally efficient heating and energy systems. We're trying not to let the perfect be the enemy of the good."

In retrofit jobs with existing houses, it's harder to get to net zero: Most older houses use too much heat. But with the right combination of solar panels and heat pumps, you can relieve the home's existing heating system of duty during most of the year, while keeping the old boiler or furnace in place to heat the house during the coldest weeks. Says Mueller: "In retrofit, most of the projects that we're seeing—and we're seeing a lot—are going in without ripping out the old heating system. It makes the projects inexpensive, and it makes the technical risks low. If you heat the center part of the living space with a mini-split, you can carry 60% of the annual load, and you can displace the oil heat by a lot for a relatively modest investment."

This month, JLC visited a job site in Portland, Maine, where a five-person ReVision Energy crew was working on that kind of job. Built in the late 1960s, the house has an oil boiler, plus a wood stove that the homeowners rely on to get them through the coldest part of the winter. But a thorough weatherization job by Efficiency Maine, the state's energy-efficiency utility, has cut the house's oil consumption significantly. Last year, ReVision put a solar water heating system on the roof, largely eliminating the boiler's water-heating role in winter. Now, the solar panel and heat pump solution that ReVision's crew installed may well be enough to take the oil boiler out of the equation completely, even for winter heat.